Typically, the insulation layer of a high frequency telecom cable is produced by mixing a nucleating agent with a mixture of high density polyethylene (HDPE) and low density polyethylene (LDPE). The foamable materials are then extruded in the presence of a physical foaming agent, likegases such as nitrogen, carbon dioxide, chlorinated fluorocarbons, freons, helium, neon, argon, krypton, xenon, and radon, which is injected into the polymer melt inside of the extruder. Nucleating agents for the foaming include azodicarbonamide (ADCA) and 4,4′-oxybisbenzenesulfonylhydrazide (OBSH), which thermally decompose in an extruder and form a number of fine nuclei in the polymer melt. However, the byproducts of the decomposed ADCA and OBSH have a high polarity which are well known to have a significant negative effect on the electrical performance (dissipation factor).
Compared to ADCA and OBSH, fluororesin powder, such as polytetrafluoroethylene (PTFE), is a nucleating agent that exhibits a significantly lesser effect on electrical performance and is free of the decomposition issues associated with ADCA and OBSH. PTFE has been and is currently used as a nucleating agent for foaming compositions for use as insulation in telecom cable but improvements are still desired, particularly with respect to dispersion of the nucleating agent within the foamable composition, i.e., the polymer matrix, and in the formation of small, uniformly sized cells within the foamed product.
The dispersion efficiency of nucleator in a polymer matrix is largely determined by the particle size and particle size distribution of the nucleator. U.S. Pat. No. 3,554,932A teaches that finely divided, solid fluororesins, such as PTFE, fluorinated ethylene-propylene (FEP), or particle carriers coated with a fluorocarbon functioned as nucleators for gas injected, foamed thermoplastic. It also teaches that the particle size should not exceed 20 microns in diameter, and it should be used in an amount from 0.01% to 2% by weight.
EP386663 teaches that fluorocarbon powder used as a nucleator should have an average particle size from 0.1 to 5 microns (μm).
U.S. Pat. No. 6,121,335 teaches a nucleator for foaming, the nucleator comprising a fluororesin powder comprising particles have a particle size of 0.1-0.5 μm in a proportion of at least 50% by number and containing particles having a particle size of not less than 5 μm in a proportion of not more than 40% by number.
U.S. Pat. No. 7,262,226 teaches using as nucleating agent tetrafluoroethylene homopolymer (PTFE) having a number average molecular weight lower than 1,000,000, preferably lower than 500,000. The nucleating agent is obtained by irradiating with gamma rays or electron beam PTFE powders produced by dispersion or suspension polymerization processes and then milling the irradiated powders. With the dispersion polymerization processes, latexes having a particle size of 0.1-0.3 micron are obtained. After coagulation the powder particle sizes increase to about 100-500 micron. The powders are irradiated with gamma rays and then milled to obtain powders having final particle sizes lower than 15 micron. With the suspension polymerization processes, powders having particle sizes of 2-5 mm are obtained. These powders are irradiated with electron beam and then milled to obtain powders having a final particle size lower than 15 micron.
None of these references teach or recognize the benefit of reducing the size of fluororesin nucleating agent agglomerates (produced dispersion polymerization) of greater than 1 micron, typically greater than 5 microns, to less than 1 micron prior to using the agents for foaming polyolefin compositions, particularly polyolefin resins comprising or consisting essentially of HDPE and LDPE. Moreover, none of these references teach or recognize the benefit of preparing the foaming composition by batch mixing (e.g., kneading) the nucleating agent of reduced size, e.g., less than 1 micron, with the polyolefin resin. These submicronucleating agents are more uniformly dispersed within the foaming composition (as opposed to a foaming composition alike in all aspects but prepared by extrusion mixing), and the foaming composition thus provides superior foaming performance and a superior foamed product, e.g., a foamed product with a desirable fine cell structure.